A variety of Coriolis-type fluid flow rate measuring devices are known and commercially available. In such devices, one or two fluid flows are typically conveyed through rotating or oscillating conduits, typically driven into oscillation by one or more electromagnetic oscillators acting at a resonant frequency of the system. This produces a Coriolis acceleration acting on the flowing fluid, and results in a Coriolis force directed perpendicular to the flow path and in ultimate opposing directions as between two legs of each conduit. This causes a sinusoidal time-varying twisting motion of the conduit which can be sensed by conventional motion sensors to generate corresponding analog sinusoidal outputs of measurable amplitude, frequency, and phase relative to a selected reference. By determining a phase difference between such sinusoidal outputs from two sensors, each sensing a motion at a different predetermined location on the conduit carrying the flow, it is possible to determine the mass flow rate of the fluid flow through the conduit.
The fundamental frequency of the system depends on the mechanical characteristics of the conduit, the density and composition of the fluid, the temperature, etc. There are also other relatively complex factors, e.g., turbulence in the flow, the presence of harmonics and/or noise generated in the system itself or brought thereto by the elements utilized to produce the oscillation of the conduit, which affect the accuracy of the measured fluid mass flow rate. As noted earlier, numerous devices and techniques for addressing some of these problems are known in the art.
Some of the earlier known Coriolis-type fluid flow rate measuring devices, for example per U.S. Pat. No. Re. 31,450, to Smith, titled "METHOD AND STRUCTURE FOR FLOW MEASUREMENT", issued on Nov. 29, 1983, determine a time difference ".DELTA.t" between sinusoidal outputs from two sensors and then obtain the desired fluid mass flow rate via a scale factor. Similar in this respect, though using different structures, are devices taught in U.S. Pat. No. 4,422,338, also to Smith, titled "METHOD AND APPARATUS FOR MASS FLOW MEASUREMENT", issued on Dec. 27, 1983, and U.S. Pat. No. 4,491,025, to Smith et al., titled "PARALLEL PATH CORIOLIS MASS FLOW RATE METER", issued on Jan. 1, 1985. The common factor in each of these devices is their exclusive use of a time interval ".DELTA.t" instead of a phase difference between two related sinusoidal outputs.
More recent developments have established that superior results, free of extraneous influences, can be obtained regardless of the flow tube geometries employed by first determining a phase difference between sinusoidal outputs from two sensors and then determining the corresponding fluid mass flow rate in devices.
U.S. Pat. No. 4,934,196, to Romano, titled "CORIOLIS MASS FLOW RATE METER HAVING A SUBSTANTIALLY INCREASED NOISE IMMUNITY", issued on Jun. 19, 1990, teaches an apparatus employing two inverted U-shaped tubular conduits driven in oscillation, which employs a Discrete Fourier Transform (DFT) evaluated at the fundamental (selected) frequency to yield corresponding frequency values specifically formed of the values of the real and imaginary components for a single frequency component. From the values of these components for both of two sensor signals, a phase angle difference therebetween is computed and the fluid mass flow rate determined therefrom. The DFT algorithm employed in Romano requires rather intense and complex computation, i.e., imposes a relatively severe computational burden on the system.
U.S. Pat. No. 4,852,410, to Corwon et al., titled "OMEGA-SHAPED CORIOLIS-TYPE MASS FLOW RATE METER", issued Aug. 1, 1989, teaches a device which employs two parallel, Omega-shaped conduits. The mass flow rate is deduced from changes in the phase difference of signals from sensors at outboard curved portions of the Omega-shaped conduits which measure the relative positions of the oscillating conduits. The principal focus in this reference is on reducing the bending stress in the conduits, reducing the response/drive frequency ratio to enable the generation of a larger phase shift for a given mass flow rate, and increasing the measurement sensitivity as compared with known U-shaped designs to permit the use of conduits of larger diameter which have a lower flow resistance and a lower pressure drop for a given fluid flow.
The present invention provides for the inclusion of elements to perform an algorithm employing a least-squares technique, to swiftly, economically and accurately determine the desired phase difference and hence the fluid mass flow rate, in a Coriolis-type flow meter system employing any conventional conduit form. The following detailed description of the invention, for convenience of explanation, describes the present invention as one incorporating the structural aspects of the conduits, the drive mechanisms, the sensors, and the omega-shaped conduits as in Corwon et al.
U.S. Pat. No. 4,852,410, to Corwon et al., is therefore expressly incorporated herein by reference for its teaching pertaining to the structures taught therein. The present invention, however, is considered to be an improvement thereover because it employs a specific highly efficient and economical algorithm to process the data in a manner that significantly reduces errors due to temperature effects, undesirable harmonics and extraneous noise in the system.